Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. Apart from photovoltaics, opto-electronic applications such as photodetectors, light-emitting diodes, and lasers on the basis of hybrid perovskite semiconductors were demonstrated in the literature. For all these applications, a precise and thorough knowledge of charge carrier dynamics is key for further steps in optimization and engineering. For thin films, the physical transport properties can differ significantly from macroscopic single crystal samples because of the impact of interfaces and the local grain structure. A microscopic measurement approach for the charge carrier mobility and diffusion properties is here needed to provide information on a level that is resembling the situation in actual devices and that can sort out the influence of grain boundaries.

We employ and demonstrate a contactless, all-optical method to visualize the charge carrier dynamics in MAPI thin films using space and time-resolved photoluminescence (PL) microscopy [1,2]. Our microscopy setup provides a spatial resolution of below 300nm, far smaller than the typical grain sizes in large crystal MAPI thin films. Key for the measurements is the observation of a delayed rise in the PL at remote positions, which is caused by the finite travel time of the charge carriers inside the material. First, we quantitatively determined the diffusion constant and separated the relevant decay channels at room temperature. We then investigated the impact of grain boundaries in large crystal MAPI thin films, and found that they pose a severe limitation to the charge carrier transport as they stop carrier diffusion [2]. Further energy transfer can then only occur in a radiative fashion. As a consequence of this restricted diffusion, the observed PL traces depend critically on the specific microscopic geometry of the crystal and the probed position inside of it.

As a next step, we investigated the temperature dependence in the range of 170K to 300K. We found that the charge carrier dynamics depend heavily on the temperature: Reduced carrier scattering at low temperatures leads to an increase in the mobility by a factor of 3 in the studied range. The decreasing Stokes shift between absorption and PL, on the other hand, results in a less efficient waveguiding effect within the crystal film. At the transition between the tetragonal and the orthorombic crystal phase, the short term carrier dynamics and transport properties show drastic spatial variations.